In a disc brake caliper, there is direct contact between the pistons that apply the hydraulic force and the back of the pad assembly that holds the friction-producing material. That friction-producing material typically is a very good conductor of heat, and the backing-plate material often is a high-strength steel that is also very heat-conductive. The piston then becomes the conductive path to the caliper body and the hydraulic fluid in the cavity behind the piston. The hydraulic fluid has a limited temperature range in which it can operate, so it must be isolated from heat transfer that would produce temperatures above that range.
Pistons can be made from many materials, such as steel, aluminum, stainless steel, titanium or engineered compounds such as phenolics and ceramics. Preferably, the material used for the piston is the same as the material used for the caliper body, so that changes in dimensions caused by thermal expansion will affect the piston the same as the body of the caliper and, in particular, so that critical dimensions will change approximately together. Doing so maintains the relationships or tolerances between key components like the piston, piston seals, piston groove and piston bore in a narrower range. This maintenance of proper tolerances adds to caliper performance and durability.
However, the current best practice is to make pistons that are used in high-temperature environments (e.g., high-performance braking systems) from more than one material, e.g., a piston body made of aluminum (where an aluminum caliper also is being used) fitted with a nose of stainless steel, titanium or ceramic. The present disclosure concerns, among other things, the configurations of such noses to improve their performance.
Conventional piston nose designs that attempt to limit thermal conductivity exist. These conventional designs include features to reduce conductive heat transfer through material choice (e.g., stainless steel, titanium or ceramic) and/or by using structures with reduced cross section.
One conventional design, shown in FIGS. 1-3 uses a simple design involving a ceramic nose 10 attached to an aluminum piston body 12. Generally speaking, this design simply relies on choice of material (ceramic) to reduce the thermal conductivity. Another conventional design, shown in FIGS. 4-6, uses a titanium nose 20 attached to an aluminum piston body 22. However, in addition to using a low-thermal-conductivity material (titanium) for the nose 20, this design also reduces thermal conductivity by shaping the nose 20 so as to reduce contact area between the piston and the brake pad, i.e., using only an outer ring 24 as the contact surface and drilling holes 25 through the ring 24. In existing implementations of piston nose 20, ring 24 is 5 or 8 millimeters (mm) high and has holes 25 that are 3 or 5 mm in diameter, respectively.
Finally, in another conventional design (not shown), heat transfer is addressed by using a thin, typically 1 mm thick, sheet of low-thermal-conductivity material, such as stainless steel or titanium, between the piston and the high-strength steel backing of the brake pad. Unfortunately, the present inventors have discovered that the problem with this solution is that this thin sheet of low-thermal-conductivity material typically becomes distorted due to high temperatures and non-uniform force imparted by the piston nose. When that occurs, the plate is no longer flat, but instead appears wavy. In addition, the present inventors have discovered that this thin layer of metallic material exhibits a spring-like characteristic, acting like a compressible spring, and thereby requiring a longer or variable distance to be traveled by the piston before direct force is exerted on the brake pad. This delay in piston force being realized on the back of the brake pad unfavorably affects brake system performance by increasing the time required to engage the brakes and the distance the brake pedal has to travel.